ABSTRACT
If all significant environmental decisions could be based on predicted or observed effects of
chemicals or other agents on organisms, there would be no need for this chapter. However,
there are many important decisions for which knowledge of organism-level effects simply is
not enough. Some species may, because of their life history or because of their greatly reduced
abundance, be more at risk than others, given the same sensitivity of individuals. Mortality to
certain individuals is unavoidable, in which case risk managers may be interested in the
amount of mortality (or growth reduction) that can be tolerated by the exposed species. It
may be necessa ry to know whet her the combined effe cts of agents affectin g severa l different
life stages may reduce the abundance of populations or lead to increased risk of extinction. It
may be important to forecast the rate of recovery of populations following an accident or a
remedial action.
